§1.1 Field of the Invention
The present invention concerns the measurement of wall shear stress (or skin friction) in a fluid flow. More specifically, the present invention concerns an optical micro-sensor which can measure wall shear stress (or skin friction) due to a flowing fluid over a solid surface with very high sensitivity and dynamic range.
§1.2 Related Art
§1.2.1 Shear Stress Measurement in Fluid Flow
The accurate measurement of wall shear stress remains a challenge in fluid mechanics. The dynamic measurement of the local wall shear stress is important not only from the standpoint of basic fluid mechanics research including high-speed and unsteady aerodynamics, but also from the perspective of dynamic flow control. Despite the long history of wall shear force measurement attempts using both direct and indirect approaches, the state of the art is still insufficient to meet all needs.
Most of the available sensors use indirect measurement techniques where the wall shear stress is inferred, through a set of assumptions, from another flow property (such as, for example, streamwise velocity or heat transfer rate) measured at or near the wall. Indirect measurement methods include hot-wire/film-based anemometry or heat flux gages surface acoustic wave sensors and laser-based velocity measurements using the Doppler Effect. However, such indirect measurement techniques lack the precision required in some applications due to the assumptions, extrapolations, and calculations used to relate the changes in flow properties to the measurement of shear stress. In addition, inferred stress measurement methods based on changes in temperature may require calibration in order to account for, and remove the effects of, the many external environmental variables which effect temperature.
Another measurement method that has been frequently used is surface oil film interferometry. Unfortunately, this approach does not provide dynamic measurement of the wall shear stress and the spatial resolution can be poor.
One of the few direct wall shear measurement techniques available is the floating element method. Using this method, the bending of a free floating, long stemmed element due to the wall shear force is measured and related to the shear stress. Recently, miniaturized sensors of this type have been produced using the available micro-electro-mechanical system (“MEMS”) technology. However, floating element sensors are essentially mechanical devices that employ moving components subjected to considerable deflections, such as a suspended plate attached to a stem that is inside an opening in the wall. This leads to certain drawbacks that include vulnerability to vibration, time response limitations, undesirable flows through gaps, and pressure and temperature gradient effects.
Another type of direct wall shear stress measurement method uses fiber-based Fabry-Perot interferometers or Fiber-Bragg gratings. These methods rely on the deflection of an optical beam to convert any change in a mechanical attribute of a structure (for example, a tip displacement) into the resonance frequency shift. This requires a precise alignment of a laser beam at the micro-fabricated mechanical system, and the use of an external photo detector array for detecting the motion of the mechanical structure. Further, the amount of deflection and/or deformation required for the measurement is too large for some applications. Thus, methods that provide a considerably finer measurement resolution than any of the other methods in this class including frequency selective fiber-based Fabry-Perot interferometers and Fiber-Bragg gratings would be useful.
In view of the foregoing, improved wall shear stress measurement techniques, and apparatus for effecting such techniques, would be useful.